The present invention relates to a method of laser welding sheet metal. In particular, the sheet steel to which the ensuing description applies solely by way of a non-limiting example, is protected by a layer of low-vaporizing-temperature materials.
As of the present, laser beam machines are known to be employed, not only for cutting flat or otherwise drawn sheet metal along given cutting lines, but also for spot or seam welding sheets together.
According to general practice, two steel sheets for laser welding are held together contacting each other as tightly as possible along the entire weld area by means of grips, so as to ensure, among other things, perfect thermal conduction between the sheets. The latter are then subjected to a laser beam which welds the sheets together by smelting the metal in the weld area swept by the said beam.
Whereas the aforementioned method has proved particularly effective for welding bare sheet steel, i.e. having no covering material protecting it against external agents, it proves inadequate when welding together metal sheets protected against external agents by using a layer of coating of low-vaporizing-temperature materials.
At this point, it should be specified that the term "low-vaporizing-temperature material" is intended to mean material, such as zinc, having a melting and vaporizing temperature considerably below that of iron.
If two thus protected metal sheets using a material such as zinc or a similar material are welded together using the same technique employed for welding bare sheet, the resulting welds invariably prove uneven and riddled with craters, faults which, regardless of whether or not the weld is endangered, involve high-cost follow-up machining for their removal, particularly when, as often occurs in the car industry where the use of zinc-plated sheet is practically universal, the said welds extend over visible parts.